The present invention relates to a copper alloy foil used in a laminate for a printed wiring board.
Printed wiring boards are used frequently in the electronic circuitry of electronic equipment. Printed wiring boards are classified broadly as either rigid laminates (i.e., rigid boards) or flexible laminates (i.e., flexible boards), according to the type of resin serving as the base material. Flexible boards, which are characterized by flexibility, are used not only for wiring in flexible regions but also as space-saving wiring material, because they can be housed within electronic equipment in a folded state. Also, because the board itself is thin, it also can be used in semiconductor package interposer applications and as an IC tape carrier in liquid-crystal displays. Although polyimide resin films and polyester resin films are used as the base material of a flexible board, polyimide is frequently used at present because of its heat resistance. Because of its conductivity, copper generally is used as the conductive material in a flexible board. In a printed wiring board, the copper foil of the copper-clad laminate is etched to form various wiring patterns, after which electronic components are connected and mounted. The term “resin” as used throughout the present specification and claims means “synthetic polymer”.
Structurally, a flexible board is either a two-layer flexible board or a three-layer flexible board. A two-layer flexible board is structured such that copper, the conductive material, is directly bonded to a resin (e.g., polyimide). On the other hand, a three-layer flexible board is structured such that a resin film (e.g., polyimide) and a copper foil, the conductive material, are bonded by means of an adhesive containing a resin such as an epoxy resin, an acrylic resin, and the like. However, a three-layer flexible board is widely used because it has good adhesion and is inexpensive. Because of such structural differences, the resin bonded with copper in a flexible board is polyimide in a two-layer flexible board, and is a resin contained in the adhesive used in a three-layer flexible board.
The adhesive used in a three-layer flexible board must have properties such as heat resistance, durability, and flexibility, so the strength and the glass transition temperature are regulated by mixing various resins. However, one consisting principally of an epoxy resin generally is used. The adhesive used in this application is a solvent or sheet adhesive that contains an elastomer or nitrile rubber that imparts flexibility as well as a curing agent (e.g., amine) in an epoxy resin, such as diglycidyl ether of bisphenol A or glycidyl ester of bisphenol A, for example. Regarding the methods used to produce a copper-clad laminate, generally a polyimide film is coated with a solvent adhesive and this is dried, after which copper foil is superposed; or a sheet adhesive is interposed between a polyimide film and copper foil, and a roll press or flat hot press is used to tentatively adhere them, after which they are adhered further by thermosetting, at 100-200° C., from several tens of minutes to as many as several hours.
In a printed wiring board, the following problem occurs: When solder is used to connect and mount electronic components, the difference in the coefficients of thermal expansion of the copper foil and the resin results in thermal stress, so if the adhesion between the copper foil and the resin is poor, peeling or delamination will occur. As a result, to improve the adhesion between the resin and copper foil used in a copper-clad laminate, the copper foil is subjected to a roughening process that forms copper particles on the surface by means of electroplating. This improves the adhesion by means of the so-called anchor effect, which roughens the surface of the copper foil and causes the copper foil to bite into the resin, thereby yielding a mechanical adhesive strength.
On the other hand, with the miniaturization, weight-saving, and enhanced functionality of electronic equipment in recent years, there has been increased demand for high-density mounting on printed wiring boards, resulting in finer pitches with narrower line or wire widths and line or wire spacing in electronic circuits. If copper foil with high surface roughness or copper foil with irregularities formed by a roughening process is used as the conductive material, when a circuit is formed by etching, etching residue containing residual copper remains in the resin, so the etching linearity drops, leading to nonuniform circuit widths. As a result, copper foil with low surface roughness is preferable, to enable finer pitches in an electronic circuit. Also, higher-frequency electrical signals are being utilized in electronic equipment (e.g., PCs, mobile telecommunications). When the electrical signal frequency exceeds 1 GHz, the skin effect (i.e., the flow of electrons only on the surface of a conductor) becomes significant, so the effect of variation in the transmission path caused by surface irregularities can no longer be disregarded. Therefore, attempts were made to reduce the surface roughness of the copper foil, because of the high-frequency characteristics.
The copper foil that serves as the conductive material in a printed wiring board is classified as a rolled copper foil or an electrolytic copper foil, depending on its production process. Electrolytic copper foil generally is produced by electrolytically depositing copper from a sulfurically acidic copper sulfate plating bath onto a titanium or stainless steel drum. Irregularities are formed on the copper foil during electrolytic deposition, so the surface roughness increases. Recently, production has begun of so-called low-profile foil, which is copper foil produced by adding additives to the plating bath and then regulating the electrolytic deposition conditions to reduce the surface roughness. Rolled copper foil is produced by using a rolling roll to plastically form it, so the surface pattern of the rolling roll is transferred to the foil surface, thereby yielding a smooth surface. Furthermore, the foil generally is less than 100 μm thick.
As aforementioned, the copper foil used in a copper-clad laminate currently is subjected to a roughening process, to improve its adhesion to the resin. For the aforesaid reasons, however, it is desirable to bond a copper foil with little surface roughness to a resin film, without performing roughening processing, so it is necessary to maintain the adhesive strength without performing roughening processing. Also, in a three-layer flexible board, measures were taken such as an attempt to coat copper foil with a silane coupling agent and so forth, in order to improve the adhesive strengths of the copper foil, which is a metal, and the adhesive, which is an organic material. However, sufficient adhesiveness was not obtained.
A copper alloy containing pure copper and small quantities of additional elements is used as the raw material of the copper foil used as the conductive material. As finer pitches are utilized in electronic circuits, the copper foil (i.e., the conductive material) thins and the circuit narrows, so two copper foil properties are desired: low DC resistance loss and high conductivity. Copper is a material with excellent conductivity, so pure copper with a high purity (above 99.9%) generally is used in the aforesaid field, where conductivity is important. However, copper's strength decreases as its purity increases, so if the copper foil is thinned, its handleability deteriorates, so a high copper foil strength is preferable.
Under such circumstances, an attempt was made to bond the copper foil (i.e., the conductive material) to a polyimide film by using an adhesive containing an epoxy resin, without subjecting it to roughening processing. However, it was ascertained that the polyimide film and the rolled high purity copper foil peeled apart easily, and peeling occurred easily at the interface between the epoxy resin and the copper. As a result, copper foil (i.e., the conductive material) not subjected to roughening processing has not reached practical application because there is a problem: The copper foil and the epoxy resin, the principal component of the adhesive, peel apart, so defects (e.g., disconnection) result.